JP2009129955A - Optical system, and atomic oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an atomic oscillator equipped with an optical system which can have a light receiving element and a light emitting element disposed side by side on the same side and facilitates module mounting by shortening a bonding wire electrically connecting the light receiving element, and is improved in S/N. <P>SOLUTION: The optical system 1 is an optical system 1 of the atomic oscillator 100 which controls an oscillation frequency by using a light absorption characteristic caused by a quantum-interference effect when the two resonant light components are incident as coherent lights differing in wavelengths, the optical system 1 including: a coherent light source 2 supplying resonant light 3 to metal atoms in a gas cell 8; a polarization splitting means 4 of transmitting p-polarized light 5 included in the resonant light 3 and converting an optical path of s-polarized light 13; a 1/4λ wavelength plate 6 converting circular polarized light into linear polarized light and vice versa; the gas cell 8 in which gaseous metal atoms are enclosed; a light guide means 10 that guides light 8 after passing through the gas cell 8 back to the gas cell 8; and a photodetector 15 that detects s-polarized light 14a having the optical path converted by the polarization splitting means 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical system for an atomic oscillator, and more particularly to a mounting technique for a light source and a light receiving element included in the optical system constituting the atomic oscillator.

An atomic oscillator using an alkali metal such as rubidium or cesium needs to keep atoms in a gas state when using energy transition of atoms, and therefore operates a gas cell in which atoms are hermetically sealed at a high temperature. . The principle of operation of an atomic oscillator is roughly divided into a double resonance method using light and microwaves, and a method using a quantum interference effect (hereinafter referred to as CPT: Coherent Population Trapping) by two types of laser light. In both cases, the amount of the light incident on the gas cell is detected by the detector provided on the opposite side to detect how much light has been absorbed by the atomic gas, so that the atomic resonance is detected and a reference signal from a crystal oscillator or the like is sent to the control system. Output is obtained in synchronization with atomic resonance. Here, in an atomic oscillator using CPT, a light emitting element, a gas cell, and a light receiving element are integrally formed to form an optical system (see Patent Document 1).
US6808064B2

However, in the configuration of the conventional optical system disclosed in Patent Document 1, the light emitting element 93, the gas cell 95, and the light receiving element 90 are vertically arranged as shown in FIG. For this reason, the bonding wire 91 for electrically connecting the light receiving element 90 arranged on the uppermost surface becomes long, and the mounting structure of the module becomes complicated, and the signal obtained from the light receiving element 90 is weak, so that the wire is used. There is also a problem that the S / N characteristic is not good because it is easily affected by superimposed noise.
In the case of an optical system in which the light emitting element 102 and the light receiving element 104 are mounted on the same substrate 112 as shown in FIG. 6 as an invention by the same applicant, the reflecting mirror 110 is provided on the gas cell 103 and the light emitting element 102 emits light. There is an optical system in which the resonance light 113 transmitted through the gas cell 103 is reflected by the reflection mirror 110 and is incident on the gas cell 103 again and is received by the light receiving element 104.

However, because of the structure, it is necessary to dispose the light emitting element 102 and the light receiving element 104 apart from each other on the same substrate. Therefore, the reflection mirror 110 must be provided with a predetermined inclination angle. As a result, there is a problem that it is necessary to adjust the angle of the reflection mirror 110 and it takes time and effort. Further, since the reflecting mirror 110 is inclined, there is a problem that the entire optical system becomes high, which is disadvantageous for downsizing.
In view of the above problems, the present invention converts the light emitted from the light emitting element into a polarization state in which the light passes back and forth through the gas cell and is orthogonally crossed by turning the light with the polarizing element shifted by ¼λ. By changing the optical path, it becomes possible to place the light receiving element and the light emitting element close to each other on the same side of the gas cell, and the bonding wire for electrically connecting the light receiving element can be shortened to facilitate module mounting. An object of the present invention is to provide an atomic oscillator including an optical system with improved S / N.

  In order to solve such a problem, the present invention is an optical system of an atomic oscillator that controls an oscillation frequency by utilizing a light absorption characteristic due to a quantum interference effect when two types of resonant light as coherent lights having different wavelengths are incident. A coherent light source that emits the resonant light, and a p-polarized light or an s-polarized light that is arranged on the outgoing side of the coherent light source and that is included in the resonant light, and passes the other polarized light in the optical path. A polarized light separating means for converting and emitting; a quarter-wave plate disposed on the outgoing side of the polarized light separating means for converting circularly polarized light into linearly polarized light or vice versa; and a gas cell enclosing gaseous metal atoms; A light guiding means that guides the light that has passed through the gas cell to the gas cell again as a return light, and passes through the gas cell and the quarter-wave plate and is converted into an optical path by the polarization separation means. And a light detection means for detecting the folded light.

The atomic oscillator of the present invention utilizes the quantum interference effect of coherent light such as laser light. In this method, two ground levels of two resonant lights irradiated simultaneously in a three-level system (for example, a Λ-type level system) in which two ground levels receive resonant light and are resonantly coupled with a common excitation level. When the frequency exactly matches the energy difference between the ground level 1 and the ground level 2, the three-level system becomes a superposition state of the two ground levels, and the excitation to the excitation level 3 stops. CPT uses this principle to detect and use a state in which light absorption in the gas cell stops when the wavelength of one or both of the two resonance lights is changed.
In the optical system of the present invention, the coherent light source and the light detection means are mounted on the same side with respect to the gas cell, and the p-polarized light contained in the resonance light from the coherent light source passes through the polarization separation means and is ¼λ wavelength. It is converted into circularly polarized light by the plate. The circularly polarized light passes through the gas cell, is folded back by the light guide means, passes through the gas cell again, is converted to linearly polarized light by the quarter-wave plate, and becomes s-polarized light, and the s-polarized light is detected by the light detecting means. . Thereby, resonance light can be reciprocated to the gas cell by one polarization separation means.

Further, the light guide means is configured such that the circularly polarized light that has passed through the gas cell is reflected by the light guide means and turns back the same optical path.
When light passes through the gas cell, a phenomenon called Doppler spread occurs. This phenomenon can be expected to be canceled by light traveling back and forth on the same optical path. Therefore, in the present invention, the light guide means is configured so that the light passing through the gas cell passes through the same optical path. This can be expected to cancel the Doppler spread.
Further, the light detection means and the coherent light source are juxtaposed on the same side with respect to the gas cell.
In the optical system of the present invention, the coherent light source and the light detection means are mounted on the same side with respect to the gas cell, and the polarization separation means is configured so that the transmitted light is received by the light detection means. As a result, the bonding wire is shortened, the signal S / N characteristic is improved, and the entire optical system can be easily mounted.

In addition, at least the gas cell or / and the light guiding means are arranged on the same optical path.
It is sufficient that the gas cell and the light guide means are configured only in the optical path through which light passes. Therefore, the gas cell and the light guide means in a place where light does not pass are not particularly necessary. Therefore, in the present invention, the gas cell or / and the light guiding means are arranged only in the portion that becomes the optical path. Thereby, the size of a gas cell and a light guide means can be minimized, and component costs can be reduced.
Further, the light guiding means is constituted by a reflecting member.
A reflective member such as a mirror is optimal for turning back the circularly polarized light incident on the light guiding means in the same optical path. Thereby, it can return to the same optical path as the optical path in which circularly polarized light entered.

The coherent light is laser light.
Ordinary light is light in which various wavelengths are mixed and the phase is random. On the other hand, laser light has good monochromaticity in wavelength and is light with a uniform phase. Coherence is defined as a measure of the stability of the wavelength and phase of light. Light with good coherence, that is, with stable wavelength and phase, can cause a quantum interference effect. In that respect, laser light is optimal.
The gaseous metal atom is rubidium or cesium.
If cesium atoms are used, a highly accurate atomic oscillator can be realized. In addition, rubidium atoms are widely spread easily. Therefore, any metal atom can be selected in consideration of the required performance and cost of the atomic oscillator.

In addition, a passive optical element that collects the light emitted from the coherent light source and corrects the light into parallel light is disposed between the coherent light source and the gas cell.
In the optical system, a passive optical element such as a lens or a wave plate is used to collect the light emitted from the coherent light source and correct it to become parallel light. This passive optical element may be disposed anywhere before entering the gas cell. Therefore, in the present invention, the passive optical element is disposed between the coherent light source and the gas cell. Thereby, light can be accurately incident on the light guide means.
Further, the optical system having the above configuration is provided in an atomic oscillator.
By adopting a structure that passes through the gas cell a plurality of times, it is possible to provide an optical system that obtains a larger EIT signal, so that a high-performance atomic oscillator with improved S / N can be provided.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment are merely illustrative examples and not intended to limit the scope of the present invention only unless otherwise specified. .
FIG. 1 is a configuration diagram of a main part of an optical system of an atomic oscillator according to an embodiment of the present invention. This optical system 1 is an optical system 1 of an atomic oscillator 100 that controls an oscillation frequency by using a light absorption characteristic due to a quantum interference effect when two types of resonant light as coherent lights having different wavelengths are incident. A coherent light source 2 that emits the resonant light 3, and a polarization separation unit 4 that is disposed on the emission side of the coherent light source 2 and passes the p-polarized light (linearly polarized light) included in the resonant light 3 and changes the optical path of the s-polarized light (linearly polarized light). The quarter-wave plate 6 disposed on the exit side of the polarization separating means 4 for converting linearly polarized light into circularly polarized light or vice versa, a gas cell 8 enclosing gaseous metal atoms, and light passing through the gas cell 8 A light guide unit 10 that leads to the gas cell 8 again, and a photodetector (light detection unit) 15 that detects s-polarized light whose optical path has been changed by the polarization separation unit 4 are provided.
The atomic oscillator 100 is further provided with a frequency control circuit 11 that controls the oscillation frequency based on the output signal of the photodetector 15.

The circularly polarized light 11 reflected by the light guiding means 10 is again incident on the gas cell 8 as reflected light to become circularly polarized light 12, and is converted into s-polarized light 13 (linearly polarized light) by the ¼λ wavelength plate 6 to separate the polarized light. Incident on the means 4. Also, since the gist of the present invention is the configuration of the optical system that constitutes the atomic oscillator, a detailed description of the frequency control of the atomic oscillator is omitted.
That is, the atomic oscillator 100 of the present embodiment uses the quantum interference effect of coherent light such as laser light. In this method, two ground levels of two resonant lights irradiated simultaneously in a three-level system (for example, a Λ-type level system) in which two ground levels receive resonant light and are resonantly coupled with a common excitation level. When the frequency exactly matches the energy difference between the ground level 1 and the ground level 2, the three-level system becomes a superposition state of the two ground levels, and the excitation to the excitation level 3 stops.
CPT uses this principle to detect and use a state in which light absorption in the gas cell stops when the wavelength of one or both of the two resonance lights is changed.
In the optical system 1 of the present invention, the coherent light source 2 and the photodetector 15 are mounted on the same side with respect to the gas cell, and the p-polarized light 5 included in the resonance light 3 from the coherent light source 2 is used as the polarization separation means 4. It passes through and is converted into circularly polarized light 7 by the quarter-wave plate 6. The circularly polarized light 7 passes through the gas cell 8 and is folded back by the light guiding means 10, passes through the gas cell 8 again as folded light, is converted into linearly polarized light by the ¼λ wavelength plate 6, and becomes s-polarized light 13, The s-polarized light 13 is separated by the polarization separation means 4 and detected by the photodetector 15. Thereby, the resonance light 3 can be reciprocated to the gas cell 8 by one polarization separation means 4.

FIG. 2 is an example for explaining a three-level system of atoms by the CPT method. The rubidium and cesium ground levels used in the atomic oscillator are divided into two kinds of ground levels by the hyperfine structure due to the nuclear spin-electron spin interaction. These ground level atoms absorb light and excite to higher energy levels.
A state in which two ground levels receive light and are resonantly coupled to a common excitation level as shown in FIG. 2 is called two-photon resonance.
In FIG. 2, since the ground level 1 (23) and the ground level 2 (24) have slightly different level energies, the resonant light also has slightly different wavelengths from the resonant light 1 (20) and the resonant light 2 (22), respectively. Different. When the frequency difference (wavelength difference) between the resonant light 1 (20) and the resonant light 2 (22) irradiated at the same time exactly matches the energy difference between the ground level 1 (23) and the ground level 2 (24), The system shown in FIG. 2 enters a superposition state of two ground levels, and excitation to the excitation level 21 stops. The CPT utilizes this principle to absorb light in the gas cell 3 (that is, to the excitation level 21) when the wavelength of one or both of the resonant light 1 (20) and the resonant light 2 (22) is changed. This is a method of detecting and using a state where the conversion is stopped. The transmitted light that passes through the gas cell 3 in a state where the light absorption is stopped is called an EIT signal.

  FIG. 3 is a diagram schematically showing the configuration of the optical system according to the first embodiment of the present invention. In this optical system 1A, a light emitting element (coherent light source 2 in FIG. 1) 30 and a light receiving element (photodetector 15 in FIG. 1) 37 are placed on a substrate 38, and each element is electrically connected to the substrate 38 by a bonding wire 25. Connected. Then, a predetermined distance is secured by the spacers 31 a and 31 b, and the coherent light 40 emitted from the light emitting element 30 is condensed thereon, converted into parallel light, or the passive optical element 32 that changes the polarization state. And a second mirror 39 for guiding the s-polarized light whose optical path has been changed by the beam splitter 33 to the substrate 38. A predetermined distance is secured by the spacers 45a and 45b, and the quarter-wave plate 36, the gas cell 34, and the first mirror 35 are overlaid thereon.

Next, the schematic operation will be described with reference to FIG. The coherent light 40 emitted from the light emitting element 30 is collected by the passive optical element 32, corrected to parallel light, and incident on a beam splitter (polarization separating means) 33. The coherent light 40 includes p-polarized light and s-polarized light, but the beam splitter 33 passes only p-polarized light. The p-polarized light 41 that has passed is converted into circularly polarized light by the quarter-wave plate 36 and enters the gas cell 34.
The gas cell 34 operates so that light absorption stops when the wavelength of one or both of the p-polarized light 41 having two wavelengths is changed. The p-polarized light 41 that has passed through the gas cell 34 is reflected by the first mirror 35 and enters the gas cell 34 again through the same optical path. Here, it is known that a phenomenon called Doppler spread occurs when light passes through the gas cell 34, and this will be supplemented. The atoms in the cell are distributed in various speeds from fast to slow. Among these, when fast-velocity atoms (excited atoms) contribute to the EIT phenomenon, the wavelength of light seen from the atoms apparently changes due to the Doppler effect, so that the detection width in the EIT phenomenon (light absorption in the gas cell stops). It is known to widen the range of wavelengths. Note that this phenomenon can be expected to be canceled by light traveling back and forth on the same optical path.

Therefore, in the present embodiment, the first mirror 35 is configured to be orthogonal to the optical path of the p-polarized light 41 so that the circularly polarized light 42 that has passed through the gas cell 34 passes through the same optical path. Thereby, it can be expected that the p-polarized light 41 is totally reflected and cancels the Doppler spread. The circularly polarized light 42 that has passed through the gas cell 34 is converted into s-polarized light (linearly polarized light) by the ¼λ wavelength plate 36 and is incident on the beam splitter 33 again. The beam splitter 33 uses the one having a characteristic of changing the optical path without passing the s-polarized light (PBS) (PBS), and the s-polarized light 43 is optically changed at a right angle and incident on the second mirror 39. , Reflected at right angles and received by the light receiving element 37. A surface emitting laser (VCSEL) is often used as the light emitting element 30 and a photodiode is often used as the light receiving element 37.
In the optical system 1A, the coherent light source 30 and the light receiving element 37 are mounted on the same side of the gas cell, and the beam splitter 33 and the second mirror 39 are configured so that transmitted light is received by the light receiving element 37. Thereby, the bonding wire 25 is shortened, the S / N characteristic of the signal is improved, and the entire optical system 1A can be easily mounted.
The p-polarized light contained in the resonance light 40 passes through the beam splitter 33 as it is, but the optical path of the s-polarized light is converted by the beam splitter 33. Therefore, in the present invention, the p-polarized light that has passed through the beam splitter 33 is converted into circularly-polarized light by passing through the quarter-wave plate 36, and the circularly-polarized light 41 passes through the gas cell 34 and is reflected by the first mirror 35. The same optical path is folded back, passes through the gas cell 34 again, is converted to s-polarized light by the quarter-wave plate 36, and is converted by the beam splitter 33. Thereby, the optical path can be separated by one beam splitter 33.

Note that the coherent light source 30 of the present embodiment uses laser light. Laser light has good monochromaticity in wavelength and has a uniform phase. Coherence is defined as a measure of the stability of the wavelength and phase of light. Light with good coherence, that is, with stable wavelength and phase, can cause a quantum interference effect. In that respect, laser light is optimal.
The gaseous metal atom used in the gas cell 34 is rubidium or cesium. By using the cesium atom used in the primary atom standard, a highly accurate atomic oscillator can be realized. In addition, since rubidium atoms used in secondary standards are easily and widely used, it is generally possible to realize a small and low-priced atomic oscillator.
Therefore, what is used for the metal atom may be selected according to the purpose of use. In this embodiment, rubidium and cesium are used, but any atom may be used as long as it has a three-level system such as a Λ-type system.
Further, although it is assumed that the p-polarized light is passed and the s-polarized light is optically path-converted by the polarization separating means, the s-polarized light may be passed and the p-polarized light is optically path-converted.

FIG. 4 is a diagram schematically showing the configuration of an optical system according to the second embodiment of the present invention. The same components will be described with the same reference numerals as in FIG. The optical system 1B is that the gas cell 46 and the first mirror 47 are configured within a range covering the optical path. That is, it is sufficient that the gas cell 46 and the first mirror 47 are configured only in an optical path through which light passes.
Therefore, the gas cell 46 and the first mirror 47 in a place where light does not pass are not particularly necessary. Therefore, in the present embodiment, the gas cell 46 and / or the first mirror 47 is arranged only in the portion that becomes the optical path. Thereby, the size of the gas cell 46 and the first mirror 47 can be minimized, and the component cost can be reduced. Further, since it is sufficient that the quarter-wave plate 36 is also in the optical path, the size can be minimized in the same manner.

It is a principal part block diagram of the optical system of the atomic oscillator which concerns on embodiment of this invention. It is a figure explaining the 3 level system of an atom by a CPT system. 1 is a diagram schematically illustrating a configuration of an optical system according to a first embodiment of the present invention. It is the figure which modeled the structure of the optical system which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structure of the conventional optical system currently disclosed by patent document 1. FIG. It is a figure which shows the structure of the optical system by the same applicant.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical system, 2 Coherent light source, 3 Resonance light, 4 Polarization separation means, 5 p polarization, 6 1/4 (lambda) wavelength plate, 7 circular polarization, 8 Gas cell, 10 Light guide means, 14 s polarization, 15 Photodetector, 16 Frequency control circuit, 100 atomic oscillator

Claims (9)

An optical system of an atomic oscillator that controls an oscillation frequency by utilizing a light absorption characteristic due to a quantum interference effect when two types of resonance light as coherent light having different wavelengths are incident,
A coherent light source that emits the resonant light;
A polarization separation unit that is arranged on the emission side of the coherent light source and transmits either one of the p-polarized light and the s-polarized light included in the resonance light, and changes the optical path of the other polarized light and emits the polarized light;
A quarter-wave plate disposed on the exit side of the polarization separation means for converting circularly polarized light into linearly polarized light or vice versa;
A gas cell containing gaseous metal atoms,
A light guiding means for guiding the light that has passed through the gas cell to the gas cell again as folded light; and
An optical system for an atomic oscillator, comprising: a light detection unit that detects the return light that has passed through the gas cell and the quarter-wave plate and has been optically path-converted by the polarization separation unit.   2. The optical system of an atomic oscillator according to claim 1, wherein the light guide means is configured such that light that has passed through the gas cell is reflected by the light guide means and turns back the same optical path.   2. The optical system of an atomic oscillator according to claim 1, wherein the light detection means and the coherent light source are juxtaposed on the same side with respect to the gas cell.   3. The optical system of an atomic oscillator according to claim 2, wherein at least the gas cell or / and the light guiding means are arranged on the same optical path.   5. The optical system of an atomic oscillator according to claim 1, wherein the light guide means is constituted by a reflecting member.   The optical system for an atomic oscillator according to claim 1, wherein the coherent light is laser light.   2. The optical system of an atomic oscillator according to claim 1, wherein the gaseous metal atom is rubidium or cesium.   8. The passive optical element that collects the light emitted from the coherent light source and corrects the light into parallel light is disposed between the coherent light source and the gas cell. 9. An optical system of an atomic oscillator described in 1.   An atomic oscillator comprising the optical system according to any one of claims 1 to 8.

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